PRE-STRESS CONSTRUCTION

Introduction:

I. Definition of Prestress:

Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. The precompression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression.

II. Basic Concept

Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.

III. Materials for prestress concrete members:

A. Cement: The cement used should be any of the following : (a) Ordinary Portland cement

conforming to IS269 (b) Portland slag cement conforming to IS455. But the slag content should not be more than 50%. (c) Rapid hardening Portland cement conforming to IS8041. (d) High strength ordinary Portland cement conforming to IS8112.

B. Concrete: Prestress concrete requires concrete, which has a high compressive strength

reasonably early age with comparatively higher tensile strength than ordinary concrete. The concrete for the members shall be air-entrained concrete composed of Portland cement, fine and coarse aggregates, admixtures and water. The air-entraining feature may be obtained by the use of either air-entraining Portland cement or an approved air-entraining admixture. The entrained air content shall be not less than 4 percent or more than 6 percent. Minimum cement content of 300 to 360 kg/m3 is prescribed for the durability requirement.

C. Steel: High tensile steel , tendons , strands or cables The steel used in prestress shall be any

one of the following:- (a) Plain hard-drawn steel wire conforming to IS1785 (Part-I & Part-III) (b) Cold drawn indented wire conforming to IS6003 (c) High tensile steel wire bar conforming to IS2090 (d) Uncoated stress relived strand conforming to IS6006

ADVANTAGES AND DISADVANTAGES:

Advantage of Prestressed Concrete:

1. The use of high strength concrete and steel in prestressed members results in lighter and slender members than is possible with RC members. 2. In fully prestressed members the member is free from tensile stresses under working loads, thus whole of the section is effective. \3. In prestressed members, dead loads may be counter-balanced by eccentric prestressing. 4. Prestressed concrete member posses better resistance to shear forces due to effect of compressive stresses presence or eccentric cable profile. 5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions. 6. Long span structures are possible so that saving in weight is significant & thus it will be economic.7. Factory products are possible.8. Prestressed members are tested before use. 9. Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse. 10. Fatigue strength is better due to small variations in prestressing steel, recommended to dynamically loaded structures.

Disadvantages of Prestressed Concrete 1. The availability of experienced builders is scanty.2. Initial equipment cost is very high. 3. Availability of experienced engineers is scanty.4. Prestressed sections are brittle5. Prestressed concrete sections are less fire resistant.

CLASSIFICATION AND TYPES:

Prestressed concrete structures can be classified in a number of ways depending upon the feature of designs and constructions.

1. Pre-tensioning: In which the tendons are tensioned before the concrete is placed, tendons are temporarily anchored and tensioned and the prestress is transferred to the concrete after it is hardened.

2. Post-tensioning: In which the tendon is tensioned after concrete has hardened. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete.

Prestressing System:

1. Pretensioning system:

In the pre-tensioning systems, the tendons are first tensioned between rigid anchor-

blocks cast on the ground or in a column or unit –mould types pretensioning bed, prior to the casting of concrete in the mould. The tendons comprising individual wires or strands are stretched with constant eccentricity or a variable eccentricity with tendon anchorage at one end and jacks at the other. With the forms in place, the concrete is cast around the stressed tendon. The system is shown in Fig. 1 below.

2. Post-tensioned system:

In post-tensioning the concrete unit are first cast by incorporating ducts or grooves to house the tendons. When the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end of the face of the member and anchored by wedge or nuts. The forces are transmitted to the concrete by means of end anchorage and, when the cable is curved, through the radial pressure between the cable and the duct. The space between the tendons and the duct is generally grouted after the tensioning operation.

Most of the commercially patented prestressing systems are based on the following principle of anchoring the tendons:

1.Wedge action producing a frictional grip on the wire.2. Direct bearing from the rivet or bolt heads formed at the end of the wire.3. Looping the wire around the concrete.

DIFFERENCES OF PRESTRESSED CONCRETE OVER REINFORECED CONCRETE:

1. In prestress concrete member steel plays active role. The stress in steel prevails whether external load is there or not. But in R.C.C., steel plays a passive role. The stress in steel in R.C.C members depends upon the external loads. i.e., no external load, no stress in steel. 2. In prestress concrete the stresses in steel is almost constant where as in R.C.C the stress in steel is variable with the lever arm. 3. Prestress concrete has more shear resistance, where as shear resistance of R.C.C is less.

4. In prestress concrete members, deflections are less because the eccentric prestressing force will induce couple which will cause upward deflections, where as in R.C.C., deflections are more. 5. In prestress concrete fatigue resistance is more compare to R.C.C. because in R.C.C. stress in steel is external load dependent where as in P.S.C member it is load independent. 6. Prestress concrete is more durable as high grade of concrete is used which are more dense in nature. R.C.C. is less durable. 7. In prestress concrete dimensions are less because external stresses are counterbalance by the internal stress induced by prestress. Therefore reactions on column & footing are less as a whole the quantity of concrete is reduced by 30% and steel reduced by about 60 to 70%. R.C.C. is uneconomical for long span because in R.C.C. dimension of sections are large requiring more concrete & steel. Moreover as self-weight increases more reactions acted on columns & footings, which requires higher sizes

SAMPLE PICTURES:

BORED PILE

Introduction:

Definition of Bored Pile:

Bored pile is another type of reinforced concrete pile which is used to support high building which has

heavy vertical load. Bored pile is a cast-in-place concrete pile where the bored piles have to be cast

on construction site, while other concrete piles like Spun Pile and Reinforced Concrete Square

Pileare precast concrete pile which they’re cast in the factory.

Normally bored piling has be to carried on those tall buildings or massive industrial complexes, which require foundations which can bear the load of thousands of tons, most probably in unstable or difficult soil conditions. Bored piling is cast by using bored piling machine which has specially designed drilling tools, buckets and grabs, it’s used to remove the soil and rock. Normally it can be drilling into 50metres depth of soil. The advantage of bored piling is its’ drilling method, little vibration and lower noise level.

The drilling method is depending on the condition of soil, piling contractor has to do soil investigation and decide which drilling technology has to be carried on. Piling contractor decide the correct drilling technology and minimize disturbance of the surrounding soil. For cohesionless soils such as sands, gravels, silts etc, whether it’s under the water table or not, the pile bore hole must be supported using steel casing or stabilizing muds such as bentonite suspension. After these, reinforcement bar will be put into the bore hole and concrete will be poured into the bore hole.

Bored piling is popular to be used in construction as a foundation especially for bridge work and tall building as well. Bored piling work has to be done by specialist bored piling contractor, normal piling contractor can’t be done without experience and knowledge about bored piles.

ADVANTAGES AND DISADVANTAGES:

Advantages:

• Easy to vary pile sizes and lengths.

• Very large diameters and lengths to support very large loads are possible.

• Soil excavated for the pile can be inspected to check it is the same as assumed in design.

• Additional reinforcement for driving is not needed.

• Pile installation is quiet compared with pile driving.

Disadvantages:

• The pile material cannot be inspected and necking is possible.

• Installing piles in groundwater can be difficult as flowing water can damage the fresh concrete.

SAMPLE PICTURES:

FLAT SLAB

Introduction:

Definition of Flat Slab:

Flat Slab- Flat slab floor is a rectangular slab directly supported by columns without beams or girders. The slab is either uniform in thickness or provided with. square symmetrical area directly above the column reinforced with bars running in two directions. The increased area directly above the column is called drop panel or simply drop. On the other hand, a flared head is employed in the construction of a flat-slab floor making a capital of the column.

When the column design is not provided with capitals, a straight flat underneath is provided in the slab throughout the system, which is called flat plate construction.

The flat slab floor system is generally economical not only in terms of materials as well as labor and is even the most suitable type of construction for industrial buildings having a wider live load and also for building in which the use of capitals are not objectionable.

A flat slab is a two-way reinforced concrete slab that usually does not have beams and girders, and the loads are transferred directly to the supporting concrete columns.

The column tends to punch through the slab in Flat Slabs, which can be treated by three methods:

a. Using a drop panel and a column capital in flat slabb. Using a drop panel without a column capital in flat slabc. Using a column capital without drop panel in flat slab

Uses of column heads :

Shear strength of flat slab is increased by using column heads.

Column heads reduce the clear or effective span, and therefore, reduce the moment in the flat slab floor

Uses of drop panels :

Drop panels increase shear strength of flat slab floor. Drop panels increase flat slab's negative moment capacity. Drop panels reduce deflection by stiffening the flat slabs.

DISADVANTAGES AND ADVANTAGES:

The advantages of the flat floor slab are:

1. Simplified formwork

2. Better light in the absence of beam and girder.

3. Advantage in height for a clear story heights.

4. Uniform surface for suspended water sprinkler system.

5. Piping and shafting

6. Absence of sharp corners

7. Better resistance to fire.

The disadvantages of flat floor slab are:

1. Span length is medium.

In flat plate system, it is not possible to have large span.

2. Not suitable for supporting brittle (masonry) partitions

3.Use of drop panels may interfere with larger mechanical ducting

4. Critical middle strip deflection

In flat slabs, the middle strip deflection may be critical.

5. Higher slab thickness

Compared to typical reinforced concrete two way slab system, the thickness of flat plat slabs are higher.

SAMPLE PICTURES

PRECAST CONSTRUCTION

Introduction.

Definition of Precast:

Precast construction is produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place. In contrast, standard concrete is poured into site-specific forms and cured on site.

TYPES OF PRECAST STRUCTURE:

Wall Panels - This type of precast structure has numerous designs depending upon the architectural. requirements. The common. shapes produced for one to four story high structures are sections having a width up to 2.40 m. They are used as curtain walls attached to columns and beams or sometimes as bearing walls. The different types of wall panels are:

1. Flat Type 2. Ribbed Type 3. Double Tee Tyoe 4. Window or Mullion Type

ROOF AND FLOOR MEMBERS

Roof and floor members are made in wide variety to suit the different conditions such as span, magnitude of load, fire ratings and appearance.

Flat Slab - Is usually 10 em thick but sometimes as thin as 7 em when used on a continuous several span having a width that ranges from 1.20 m to 2.40 m with a length up to 11.00 meters.

Hollow Plank - Is a lightweight member that covers a longer span made by extrusion Jn special machine with a thickness that ranges from 10 em to 20 em and the width ranges from .60 to 1.20 m used on roof having a span from 5.00 m to 10.00 m and also on floor with 3.50 to 7.00 m span which could be augmented to 9.00 m when 5 em topping is applied to act ethnographically with the hollow plank.

Double Tee- Are the most widely used shapes for longer span having a depth from 4.00 to 6.50 m generally used on roof having a span up to 18 m when a topping of at least 5 .em is applied to act monolithically with the precast members. It could be used on floor to a span up to 15 meters depending upon the load and deflection ·requirements.

Single Tee - Are used for roofing having a span up to 30 meters and more. The flange of the Tee constitute the floor or roof slab.

PRECAST BEAMS

The shape of precast beams depends upon the manner of framing. The various shapes are:

1. Rectangular Beam - Where the floor and roof members are supported on top of the beam.

2. Ledger Beam - Is designed to reduce the height of the floor and roof construction.

3. L beam - To provide bearing, the beam is designed in a form of L.

4. AASHTO Bridge Girder - Named after the American Association of State Highway and Transportation Officials.

PRECAST COLUMN

Precast column sizes are from .30 x .30m to .60 x .60 meters. In a multi-story construction, the columns are made continuous up to four stories wherein corbels are used to provide bearing for the beam. Tee column is sometimes used to support directly double Tee floor members without the use of intermediate members.

SHAPE OF PRESTRESSED STRUCTURE

The common shapes .of prestressed structural members are:

1. Double TEE - Is considered as the most widely used section for prestressed construction with a flat surface having a width that ranges from 1.20 to 2.40 'meters wide. The thickness depends upon the requirements while the span can extend up to 18 meters.

2. Single TEE - Is normally used for longer span up to 36 ·meters with heavier loads.

3. I·Section -· Is widely used for bridges. roof, girders up to 36 meters span.

4. Channel Slab- Is used for floors in the intermediate span.

5. Box Girder - Is used on bridges of intermediate and major span.

6. Inverted T- section - Provides a bearing ledge to carry the precast deck members having a perpendicular direction of span.

ADVANTAGES AND DISADVANTAGES OF PRECAST

Advantages:

1. Very heavy members.

2. Connections may be difficult

3. Limited building design flexibility

4. Skilled workmanship is required in the application of the panel in the site.

5. Economics of scale demand regularly shaped buildings.

Disadvantages:

1. Very rapid of erection.

2. Good quality control

3. Rapid construction on site

4. High quality

T- BEAMS

Definition of T -BEAM

- is used in construction, is a load-bearing structure of reinforced concrete, wood or metal, with a t-shapedcross section. The top of the t-shaped cross section serves as a flange or compression member in resisting compressive stresses. The web (vertical section) of the beam below the compression flange serves to resist shear stress and to provide greater separation for the coupled forces of bending.

The T-beam has a big disadvantage compared to an I-beam because it has no bottom flange with which to deal with tensile forces. One way to make a T-beam more efficient

structurally is to use an inverted T-beam with a floor slab or bridge deck joining the tops of the beams. Done properly, the slab acts as the compression flange.

STEEL T- BEAMS

Steel T-beams manufacturing process includes: hot rolling, extrusion, plate welding and pressure fitting. A process of large rollers connecting two steel plates by pinching them together called pressure fitting is a common process for non-load bearing beams. The reality is that for most roadways and bridges today, it is more practical to bring concrete into the design as well. Most T-beam construction is not with steel or concrete alone, but rather with the composite of the two, namely, reinforced concrete. Though the term could refer to any one of a number of means of reinforcement, generally, the definition is limited to concrete poured around rebar. This shows that in considering materials available for a task, engineers need to consider the possibility that no one single material is adequate for the job; rather, combining multiple materials together may be the best solution. Thus, steel and concrete together can prove ideal.

DOUBLE T- BEAM

A double-T beam or double tee beam is a load-bearing structure that resemble two T-beams connected to each other. Double tees are manufactured from prestressed concreteusing pretensioning beds of about 200-foot (61 m) to 500-foot (150 m) long. The strong bond of the flange (horizontal section) and the two webs (vertical members) creates a structure that is capable of withstanding high loads while having a long span. The typical sizes of double tees are up to 15 feet (4.6 m) for flange width, up to 5 feet (1.5 m) for web depth and up to 80 feet (24 m) or more for span length.

MINDANAO UNIVERSITY OF SCIENCE AND TECHNOLOGY

COLLEGE OF ENGINEERING AND ARCHITECTURE

Building Tech 5 (Alternative Building Construction System)

Narrative Report

METHODS OF BUILDING CONSTRUCTION

Submitted by:

Iano AL-O Adorable

BS- ARCH 4B

Submitted to:

Archt. Rey Galua

Instructor